Question Number 38456 by maxmathsup by imad last updated on 25/Jun/18
$${calculate}\:\sum_{{n}=\mathrm{1}} ^{\infty} \:\left(−\mathrm{1}\right)^{{n}} \frac{{cos}\left({nx}\right)}{{n}^{\mathrm{2}} }\:\:{and}\:\sum_{{n}=\mathrm{1}} ^{\infty} \:\left(−\mathrm{1}\right)^{{n}} \:\frac{{sin}\left({nx}\right)}{{n}^{\mathrm{2}} } \\ $$